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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">> A little bit of curiously. What if user doesn't restore the OldRM like this:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">> double someFunc(double A, double B, bool ForceRoundUp) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">> #pragma STDC FENV_ACCESS ON<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">> double Result;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">> if (ForceRoundUp) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">> int OldRM = fegetround();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">> fesetround(FE_UPWARD);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">> Result = A/B;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">> <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">> // fesetround(OldRM);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">> } else {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">> Result = A/B;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">> return Result;
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">> Are we still going to generate llvm.begin.local.roundingmode/llvm.end.local.roundingmode?<o:p></o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></a></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">I was actually thinking that fegetround and fesetround would always be translated into llvm.get.roundingmode and llvm.set.roundingmode at least initially, because
it’s a bit of trouble to prove that they are being used to implement the local rounding mode idiom. Certainly recognizing that kind of usage is more work than a front end should be doing.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">I would propose the begin/end local rounding mode intrinsics as a convenience for cases where some user (probably never clang) specifically knew that this was
the intended behavior. I believe Wei has a use case like that, and I’ve also spoken to someone working on the Julia language who would like to be able to do this.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">That does still leave the problem of what happens if there is a function call inside a pair of begin/end local rounding mode intrinsics. I’m not entirely sure
the best way to handle this. Do we try to restrict calls that might change the rounding mode or do we leave it up to the front end and/or user to make sure things are safe.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">As for the C/FENV_ACCESS case, I seriously feel like the way that pragma is defined leaves users with a huge amount of room to shoot themselves in the foot.
If I’m reading the specifications correctly, it says that changing from a scope with FENV_ACCESS on to a scope with FENV_ACCESS off or vice versa without the FP environment in its default state is undefined behavior. At some point it would probably be nice
to have a sanitizer that checks this.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">-Andy<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext"> Hal Finkel [mailto:hfinkel@anl.gov]
<br>
<b>Sent:</b> Friday, November 03, 2017 5:46 PM<br>
<b>To:</b> </span><span style="font-size:11.0pt;font-family:"MS Gothic";color:windowtext">陳韋任</span><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext"> <chenwj.cs97g@g2.nctu.edu.tw>; Kaylor, Andrew <andrew.kaylor@intel.com><br>
<b>Cc:</b> llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> Re: [llvm-dev] FW: clarification needed for the constrained fp implementation.<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p><o:p> </o:p></p>
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<p class="MsoNormal">On 11/03/2017 05:26 PM, <span style="font-family:"MS Mincho"">
陳韋任</span> via llvm-dev wrote:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">2017-11-04 4:29 GMT+08:00 Kaylor, Andrew via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>>:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Copying the list on a discussion of potentially general interest….</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><a name="m_4695229196957410940__MailEndCompose"><span style="color:#1F497D"> </span></a><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b>From:</b> Kaylor, Andrew
<br>
<b>Sent:</b> Friday, November 03, 2017 1:11 PM<br>
<b>To:</b> 'Ding, Wei' <<a href="mailto:Wei.Ding2@amd.com" target="_blank">Wei.Ding2@amd.com</a>>; Sumner, Brian <<a href="mailto:Brian.Sumner@amd.com" target="_blank">Brian.Sumner@amd.com</a>>; Arsenault, Matthew <<a href="mailto:Matthew.Arsenault@amd.com" target="_blank">Matthew.Arsenault@amd.com</a>><br>
<b>Subject:</b> RE: clarification needed for the constrained fp implementation.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Hi Wei,</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">I’ve been meaning to write something up for discussion on the LLVM Dev list about this. I hope you don’t mind if I copy the list now to accomplish that
while also answering your questions. Eventually I create a document describing this in more detail and less formally than the language definition.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Basically, the “constraints” in the constrained FP intrinisics are constraints on the optimizer. They are a way of telling the optimizer what it can
and cannot assume about rounding mode and FP exception behavior. By default, the optimizer assumes that the rounding mode is round-to-nearest and that FP exceptions are being ignored. If the user code is going to do anything that invalidates these assumptions,
then we need a way to make the optimizer stop assuming that. That’s what the intrinisics do. Because most passes don’t recognize the intrinisics, they can’t do anything with the operations they represent and therefore can’t make any assumption about them.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">The intrinsics are not intended to do anything to change the rounding mode or FP exception handling state. I have an idea in mind for some additional
intrinsics that would provide a way to control the FP environment. There are already some target-specific mechanisms for doing that, but I’d like to have something that’s target independent. I’ll say more about this in a minute.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">I mentioned in my review comments that my work on this has been motivated by the STDC pragmas, and I think if I explain that it might make the semantics
of the intrinsics seem a little more natural. The primary pragma I have in mind here is the “STDC FENV_ACCESS” pragma. I believe this is part of the C99 standard, but compiler support for it is still mostly (if not entirely) missing. For instance, if you
try to use this pragma with clang you will get a message telling you that the pragma isn’t supported and it will have no other effect. We want to change that.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">Basically, what the “STDC FENV_ACCESS” pragma does is provide programmers with a way to tell the compiler that the program might change the FP environment.
This pragma represents a setting that has only two states -- on and off. The default setting of this state is documented as being implementation defined. In clang the default state will be off. The C99 standard states that accessing the FP environment (testing
FP status flags, changing FP control modes, etc.) when FENV_ACCESS is off is undefined behavior. The C99 standard provides library calls to access the environment (fesetround, fegetround, fetestexcept, etc.) but you can only safely use these if you have set
FENV_ACCESS to the “on” state. A typical usage might look like this:</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">#include <fenv.h></span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">double someFunc(double A, double B, bool ForceRoundUp) {</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> #pragma STDC FENV_ACCESS ON</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> double Result;</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> if (ForceRoundUp) {</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> int OldRM = fegetround();</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> fesetround(FE_UPWARD);</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> Result = A/B;</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> fesetround(OldRM);</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> } else {</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> Result = A/B;</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> }</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> return Result;</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">}</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">... <o:p></o:p></span></p>
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<p class="MsoNormal">abridge <o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"> ...<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><o:p> </o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">What I’m thinking is that we need something like this:</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">void llvm.set.roundingmode(i32 mode)</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">i32 lllvm.get.roundingmode()</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">These would then get translated during instruction selection to target-specific instructions, which is equivalent to what fesetround() and fegetround()
do. But I think it would also be useful to have something like this:</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">void llvm.begin.local.roundingmode(i32 mode)</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D">void llvm.end.local.roundingmode()</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">A little bit of curiously. What if user doesn't restore the OldRM like this<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal">double someFunc(double A, double B, bool ForceRoundUp) {<br>
#pragma STDC FENV_ACCESS ON<br>
double Result;<br>
if (ForceRoundUp) {<br>
int OldRM = fegetround();<br>
fesetround(FE_UPWARD);<br>
Result = A/B;<br>
<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">// <o:p></o:p></span></p>
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<p class="MsoNormal">fesetround(OldRM);<br>
} else {<br>
Result = A/B;<br>
}<br>
return Result; <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">}<o:p></o:p></p>
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<p class="MsoNormal"><br clear="all">
<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Are we still going to generate llvm.begin.local.roundingmode/llvm.end.local.roundingmode?<o:p></o:p></span></p>
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I think that, however we do this, we'll need a way of dealing with "live in" and "live out" FP-environment state. Moreover, any external call can change these as well (unless we prove/know that it doesn't). Translating these calls into intrinsics so that the
optimizer can reason about them seems like a reasonable plan.<br>
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-Hal<br>
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<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Regards,<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">chenwj<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">-- <o:p></o:p></p>
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<p class="MsoNormal">Wei-Ren Chen (<span style="font-family:"MS Mincho"">陳韋任</span>)<br>
Homepage: <a href="https://people.cs.nctu.edu.tw/%7Echenwj" target="_blank">https://people.cs.nctu.edu.tw/~chenwj</a><o:p></o:p></p>
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<br>
<br>
<o:p></o:p></p>
<pre>_______________________________________________<o:p></o:p></pre>
<pre>LLVM Developers mailing list<o:p></o:p></pre>
<pre><a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a><o:p></o:p></pre>
<pre><a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev">http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a><o:p></o:p></pre>
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<o:p></o:p></p>
<pre>-- <o:p></o:p></pre>
<pre>Hal Finkel<o:p></o:p></pre>
<pre>Lead, Compiler Technology and Programming Languages<o:p></o:p></pre>
<pre>Leadership Computing Facility<o:p></o:p></pre>
<pre>Argonne National Laboratory<o:p></o:p></pre>
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